1. Field of the Invention
This invention relates to a method and a drilling fluid/cement slurry composition for cementing a wellbore. More specifically the present invention relates to a method and composition for displacing drilling fluid from an annular space in a wellbore and sealingly occupying the space with the drilling fluid/cement composition.
2. Description of the Prior Art
Techniques and methods are well known for drilling and completing wells, particularly oil and gas wells which are drilled from the surface of the earth into subterranean formations containing oil and gas. Typically a bore hole is drilled from the earth's surface to the fluid-containing geologic formation to permit recovery of the fluid material contained therein.
Such wells are typically drilled by positioning a conductor tube or casing from the earth's surface a short distance into the earth and thereafter initiating drilling operations through the conductor tube. Drilling operations are conducted by maintaining a weight on a drill bit which is rotated by a drill string which is a length of pipe extending from the surface to the drill bit. Drilling fluids (frequently referred to as drilling muds) are passed through the drill string and the drill bit into the wellbore where they perform a variety of functions such as removing drill cuttings upwardly around the outside of the drill string to the surface.
Drilling fluids used during drilling of the wellbore are generally classified on the basis of their principal component. When the principal component is a liquid (water or oil) the term "mud" is applied to a suspension of solids in the liquid. The most common drilling fluids contain water as the principal component and are referred to as water based drilling fluids or water muds.
Water based drilling fluids vary widely in composition and properties and many have been classified based upon their principal ingredients. Some common classifications of water-based drilling fluids are fresh water muds, low solid muds, spud muds, salt water muds, lime muds, gyp muds and CL-CLS muds.
In addition to water, drilling fluids often contain several known additives which perform various functions in the drilling fluid system. Among the most common additives are materials which increase density such as barite and hematite and viscosifiers or thickening agents such as clays (usually bentonite). The term drilling fluid as used herein includes spacer fluids which typically comprise a drilling fluid containing no viscosifiers or densifiers.
Many water-based drilling fluids are also formulated to contain one or more polymeric additives which serve to reduce the rate of filtrate loss, stabilize clays, flocculate drilled solids, increase carrying capacity, emulsify, lubricate and the like. Among the most commonly employed polymeric additives are starches, guar gum, xanthan gum, sodium carboxyl-methylcellulose (CMC), hydroxyethylcellulose (HEC), carboxyl methyl hydroxyethyl cellulose (CMHEC) and synthetic water dispersable polymers such as acrylics and alkylene-oxide polymers.
Such drilling fluids also frequently contain materials referred to broadly as "dispersants". Materials conventionally used as dispersants are classified as plant tannins, polyphosphates, lignitic materials, synthetic water dispersable polymers and lignosulfonates. Such drilling fluids are described in considerable detail in U.S. Pat. No. 5,030,366, "Spacer Fluids", issued Jul. 9, 1991 to Wilson et al; U.S. Pat. No. 5,113,943, "Spacer Fluids", issued May 19, 1992 to Wilson et al; and U.S. Pat. No. 5,292,367, "Dispersant Compositions for Subterranean Well Drilling and Completion", issued Mar. 8, 1994 to Bloys et al; all of which are hereby incorporated in their entirety by reference.
After the wellbore has been drilled, the casing is secured in position in the wellbore to ensure permanence of the wellbore and to control the entry of fluids into the casing from formations penetrated by the wellbore. The well casing is typically cemented in place by pumping a cement slurry downwardly through the casing, out through the open lower end of the casing at the well bottom and then upwardly around the casing in the annular space between the outer wall of the casing and the inner wall of the wellbore. Frequently a spacer fluid is pumped downwardly through the casing ahead of the cement slurry to form a buffer between and prevent the contact of the drilling fluid and the cement slurry which are typically incompatible fluids.
The drilling process which produces the wellbore will usually leave behind on the wall of the wellbore a drilling fluid filter cake of mud-like material. This material is detrimental to the formation of proper bonding between the cement composition and the walls of the wellbore.
Desirably the drilling fluid filter cake is removed from the wellbore wall and replaced by the cement slurry to permit the formation of a solid layer of hardened, cured and bonded cement between the casing and the walls of the wellbore. It is well known that the removal of the drilling fluid filter cake is greatly enhanced by injecting the cement slurry, spacer fluid or the like into the wellbore under turbulent flow conditions.
Suitable spacers for this purpose have been disclosed in U.S. Pat. Nos. 5,030,366; 5,113,943 and 5,292,367 previously incorporated by reference. In cementing oil and gas wells, rather than displacing and removing the drilling fluid during cement slurry placement, it has been proposed to convert the drilling fluid into a cementious slurry for cementing casing or tubing in place or otherwise stabilizing or protecting the casing by sealing the formation in the vicinity of the wellbore using a dispersant such as sulfonated styrene maleic anhydride (SSMA). Such a method is disclosed in U.S. Pat. No. 4,883,125, "Cementing Oil and Gas Wells Using Converted Drilling Fluid," issued Nov. 28, 1989 to Wilson et al; which is hereby incorporated in its entirety by reference.
Drilling mud constitutes a waste stream or a waste material when the drilling operation is concluded unless it can be used in another well drilling operation or the like. Further, the disposal of drilling muds is subject to increasingly severe environmental restrictions. Accordingly, it is desirable that the amount of surplus drilling fluid remaining for disposal at the end of the well completion be minimized. As a result there has been considerable interest in using the drilling fluid in combination with cementious materials to produce a cementitious slurry-drilling fluid mixture for use in cementing the casing in the well.
The conversion of drilling fluid or "mud" to a cement slurry is subject to several operational problems and undesirable compositional changes. For instance, the addition of cementitious materials such as mixtures of lime, silica and alumina, lime and magnesia, silica, alumina and iron oxide, cement materials such as calcium sulphate and Portland cements, pozzolanic materials such as ground slag, fly ash and the like to drilling fluids can substantially increase the viscosity of the fluid mixture and result in severe flocculation. Efforts to circulate such mixtures through a wellbore can result in highly unsatisfactory circulation rates, plugging of the wellbore annulus, breakdown of the earth formation in the vicinity of the wellbore, failure of the cement slurry to properly mix and the like.
These problems can be overcome by the addition of a dispersant composition which disperses both the drilling fluid and the cement slurry mixture. However, conventional drilling fluid dispersants do not necessarily disperse cement slurries and conventional cement slurry dispersants do not necessarily disperse drilling fluids. In addition, conventional dispersants which disperse both drilling fluids and cement slurries do not necessarily disperse mixtures of drilling fluids and cement slurries.
It is known in the art that hydroxypropyl acrylate-acrylic acid copolymer, sulfonated vinyl toluene-maleic anhydride copolymer, sulfonated vinyl toluene-maleic anhydride copolymer (SVT-MA), sulfonated methyl, ethyl, or phenyl polyacrylamide, and copolymers of ethylenic acids, acrylamides and ethylenic esters of phosphoric acid, and the sodium salts thereof act as dispersants or deflocculants in drilling fluids.
As further disclosed in U.S. Pat. No. 5,292,367, polymers including hydroxypropyl acrylate-acrylic acid copolymer, sulfonated vinyl toluene-maleic anhydride copolymer, sulfonated vinyl toluene-maleic acid copolymer, sulfonated methyl, ethyl or phenyl polyacrylamide, copolymers of ethylenic acids, acrylamides and ethylenic esters of phosphoric acid, acrylic acid polymers, acrylamide polymers, and sulfonated and/or phosphonated copolymers of acrylic acid and acrylamide are effective to disperse mixtures of drilling fluids and cement slurries. Unfortunately, the preferred polymers (SSMA and SVT-MA) are in short supply and are not readily available commercially in the quantities necessary for drilling operations.
Accordingly, continuing efforts have been directed to the development of alternate methods and other dispersants for use to disperse mixtures of drilling fluids and cement slurries to produce drilling fluid/cement slurry compositions for use in cementing oil wells.